A macroscopic depletion method for pebble-bed HTR fuel-cycle analysis with AGREE

Because of the low-carbon generation nature of nuclear energy and its reliability to provide base load electricity, there is a recognized need to consider nuclear reactors as a future source of energy. However, emerging technologies such as the next generation of advanced small modular reactors are being assessed for safety performance which includes the validation and verification of computer codes used to model and simulate reactor transient behavior during anticipated accident scenarios. Most of the current generation of reactor safety codes have been validated primarily for large light-water reactor systems and the unique features and physics of a small modular high-temperature gas-cooled pebble bed reactors can differ significantly from the LWR and require a different set of experimental facilities and a different range of validation data. The objective of this paper was to extend that validation of the AGREE HTR safety analysis code to fuel depletion using the HTR-200 benchmark problem. The depletion capability was developed for AGREE using a quasi-batchwise fuel loading method and applied to the once-through-then-out (OTTO) fuel pass to achieve an equilibrium condition. The depletion analysis is performed using a two-step macroscopic cross section approach for full-core depletion which was implemented in AGREE. The two-step method used both Monte Carlo and deterministic reactor physics methods in which the Serpent Monte Carlo code was generated region-wise cross sections for the AGREE deterministic full core depletion. The spatially homogenized and energy condensed macroscopic cross-sections accounted for the effects of both instantaneous and history variables to include fuel burnup. Validation was performed by comparing AGREE results with both the legacy HTR code VSOP results reported in the benchmark reference documentation and the full-core, temperature-dependent Monte Carlo Serpent simulation results.